PML Symptoms: What to Watch For and When to Act

When the progressive multifocal leukoencephalopathy, a rare and serious brain disease caused by the JC virus reactivating in people with weakened immune systems. Also known as PML, it doesn’t show up in routine scans and often gets missed until it’s advanced. Most people carry the JC virus without issues—it’s harmless until your immune system takes a hit. That’s when PML symptoms start creeping in, usually in people on long-term treatments for MS, lymphoma, or autoimmune diseases.

Early signs are easy to ignore. You might feel a little weak on one side, stumble more than usual, or notice your vision is blurry in one eye. These aren’t just "getting older" or "stress"—they’re red flags. As PML spreads, it eats away at the white matter in your brain, and symptoms get worse fast: slurred speech, trouble swallowing, loss of coordination, or sudden confusion. If you’re on drugs like natalizumab, rituximab, or efalizumab, and you notice any of this, don’t wait. PML moves quicker than most infections, and every day counts.

The JC virus, a common polyomavirus that stays dormant in kidneys and bone marrow until immune suppression triggers it. Also known as John Cunningham virus, it’s behind every case of PML. You can’t test for it before it activates, but knowing your risk helps. If you’ve been on immunosuppressants for over a year, talk to your doctor about monitoring. Blood tests won’t catch PML—only MRI scans and spinal fluid analysis can. And even then, it’s tricky. Many patients are misdiagnosed with stroke or multiple sclerosis because the symptoms look so similar.

There’s no cure for PML, but stopping the immune-suppressing drug early can slow or even stop it. That’s why recognizing symptoms early matters more than anything. If you’re on one of these meds, keep a symptom journal. Note any new numbness, vision changes, or memory slips. Bring it to your neurologist. Don’t assume it’s nothing. PML is rare, but when it hits, it hits hard. The posts below cover real cases, drug risks, and what doctors look for when PML is suspected—so you know what questions to ask and what tests to push for.